What happened to the medium ground finch population on Daphne Major island in the late 1970s?

The arrival 36 years ago of a strange bird to a remote island in the Galápagos archipelago has provided direct genetic evidence of a novel way in which new species arise.

On Nov. 23 in the journal Science, researchers from Princeton University and Uppsala University in Sweden report that the newcomer belonging to one species mated with a member of another species resident on the island, giving rise to a new species that today consists of roughly 30 individuals.

The study comes from work conducted on Darwin’s finches, which live on the Galápagos Islands in the Pacific Ocean. The remote location has enabled researchers to study the evolution of biodiversity due to natural selection under pristine conditions.

The direct observation of the origin of this new species occurred during field work carried out over the last four decades by B. Rosemary Grant and Peter Grant, a wife-and-husband team of scientists from Princeton, on the small island of Daphne Major.

"The novelty of this study is that we can follow the emergence of new species in the wild," said B. Rosemary Grant, a senior research biologist, emeritus, and a senior biologist in the Department of Ecology and Evolutionary Biology. "Through our work on Daphne Major, we were able to observe the pairing up of two birds from different species and then follow what happened to see how speciation occurred."

In 1981, a graduate student working with the Grants on Daphne Major noticed the newcomer, a male that sang an unusual song and was much larger in body and beak size than the three resident species of birds on the island.

"We didn't see him fly in from over the sea, but we noticed him shortly after he arrived. He was so different from the other birds that we knew he did not hatch from an egg on Daphne Major," said Peter Grant, the Class of 1877 Professor of Zoology, Emeritus, and a professor of ecology and evolutionary biology, emeritus.

The researchers took a blood sample and released the bird, which later bred with a resident medium ground finch of the species Geospiz fortis, initiating a new lineage. The Grants and their research team followed the new "Big Bird lineage" for six generations, taking blood samples for use in genetic analysis.

In the current study, researchers from Uppsala University analyzed DNA collected from the parent birds and their offspring over the years. The investigators discovered that the original male parent was a large cactus finch of the species Geospiza conirostris from Española island, which is more than 100 kilometers (about 62 miles) to the southeast in the archipelago.

The remarkable distance meant that the male finch was not able to return home to mate with a member of his own species and so chose a mate from among the three species already on Daphne Major. This reproductive isolation is considered a critical step in the development of a new species when two separate species interbreed.

The offspring were also reproductively isolated because their song, which is used to attract mates, was unusual and failed to attract females from the resident species. The offspring also differed from the resident species in beak size and shape, which is a major cue for mate choice. As a result, the offspring mated with members of their own lineage, strengthening the development of the new species.

Researchers previously assumed that the formation of a new species takes a very long time, but in the Big Bird lineage it happened in just two generations, according to observations made by the Grants in the field in combination with the genetic studies.

All 18 species of Darwin’s finches derived from a single ancestral species that colonized the Galápagos about one to two million years ago. The finches have since diversified into different species, and changes in beak shape and size have allowed different species to utilize different food sources on the Galápagos. A critical requirement for speciation to occur through hybridization of two distinct species is that the new lineage must be ecologically competitive — that is, good at competing for food and other resources with the other species — and this has been the case for the Big Bird lineage.

"It is very striking that when we compare the size and shape of the Big Bird beaks with the beak morphologies of the other three species inhabiting Daphne Major, the Big Birds occupy their own niche in the beak morphology space," said Sangeet Lamichhaney, a postdoctoral fellow at Harvard University and the first author on the study. "Thus, the combination of gene variants contributed from the two interbreeding species in combination with natural selection led to the evolution of a beak morphology that was competitive and unique."

The definition of a species has traditionally included the inability to produce fully fertile progeny from interbreeding species, as is the case for the horse and the donkey, for example. However, in recent years it has become clear that some closely related species, which normally avoid breeding with each other, do indeed produce offspring that can pass genes to subsequent generations. The authors of the study have previously reported that there has been a considerable amount of gene flow among species of Darwin’s finches over the last several thousands of years.

One of the most striking aspects of this study is that hybridization between two distinct species led to the development of a new lineage that after only two generations behaved as any other species of Darwin’s finches, explained Leif Andersson, a professor at Uppsala University who is also affiliated with the Swedish University of Agricultural Sciences and Texas A&M University. "A naturalist who came to Daphne Major without knowing that this lineage arose very recently would have recognized this lineage as one of the four species on the island. This clearly demonstrates the value of long-running field studies," he said.

It is likely that new lineages like the Big Birds have originated many times during the evolution of Darwin’s finches, according to the authors. The majority of these lineages have gone extinct but some may have led to the evolution of contemporary species. "We have no indication about the long-term survival of the Big Bird lineage, but it has the potential to become a success, and it provides a beautiful example of one way in which speciation occurs," said Andersson. "Charles Darwin would have been excited to read this paper."

The study was supported by the Galápagos National Parks Service, the Charles Darwin Foundation, the National Science Foundation, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council.

The study, "Rapid hybrid speciation in Darwin's finches," by Sangeet Lamichhaney, Fan Han, Matthew T. Webster, Leif Andersson, B. Rosemary Grant and Peter R. Grant, was published in the journal Science on Nov. 23.

Uppsala University contributed to the content of this press release.

Photo: Galápagos finch, by Mike's Birds from Riverside, CA, US, CC BY-SA 2.0 , via Wikimedia Commons.

Author’s note: Are Darwin’s finches “a particularly compelling example of speciation” as well as “evolution in action”? In a series of posts, I offer some notes on the question of whether macroevolution is happening on the Galápagos Islands. Please find the full series here.

It may perhaps sound a bit confusing and contradictory when we read (Lamichhaney et al. in Nature 2015, referred to in my last post) that, “After a year of drought the finches with slightly larger beaks survived earlier than those with smaller beaks,” and also: “When the weather is dry, bigger-beaked birds do better. When the rain returns, smaller-beaked birds return to previous levels,” etc., and that during the time of the drought: “the researchers found that the large-beak HMGA2 variant was more common in birds that starved to death, while the small-beak variant was more common in birds that survived” (emphasis added). 

Solving the Riddle

The solution to the riddle?

First phase (for the Wikipedia article, see here):

During the rainy season of 1977 only 24 millimeters of rain fell. Two of the main finch species were hit exceptionally hard and many of them died. The lack of rain caused major food sources to become scarce, causing the need to find alternative food sources. The smaller, softer seeds ran out, leaving only the larger, tougher seeds. The finch species with smaller beaks struggled to find alternate seeds to eat. The following two years suggested that natural selection could happen very rapidly. Because the smaller finch species could not eat the large seeds, they died off. Finches with larger beaks were able to eat the seeds and reproduce. The population in the years following the drought in 1977 had “measurably larger” beaks than had the previous birds.

Second phase (for the Wikipedia article that is the source for this and subsequent quotations, see here): 

Over the course of 1982–1983, El Niño brought a steady eight months of rain. In a normal rainy season Daphne Major usually gets two months of rain. The excessive rain brought a turnover in the types of vegetation growing on the island. The seeds shifted from large, hard to crack seeds to many different types of small, softer seeds. This gave birds with smaller beaks an advantage when another drought hit the following year. Small-beaked finch could eat all of the small seeds faster than the larger beaked birds could get to them.

Third phase:

In 2003, a drought similar in severity to the 1977 drought occurred on the island.

Events Repeated?

Now you would expect that the events that happened in 1977 would be repeated, namely: 

Because the smaller finch species could not eat the large seeds, they died off. Finches with larger beaks were able to eat the seeds and reproduce. The population in the years following the drought in 1977 had “measurably larger” beaks than had the previous birds.

But far from it. Instead, the following was observed:

Following the drought, the medium ground finch population had a decline in average beak size, in contrast to the increase in size found following the 1977 drought.

Note the Subjunctive

Why? The authors offer the following selectionist explanation, which is nevertheless uncertain. Note the repeated use of the subjunctive: 

…in the time between the droughts (beginning in late 1982), the large ground finch (Geospiza magnirostris) had established a breeding population on the island. This species has diet overlap with the medium ground finch (G. fortis), so they are potential competitors. The 2003 drought and resulting decrease in food supply may have increased these species’ competition with each other, particularly for the larger seeds in the medium ground finches’ diet. This was hypothesized to be due to the presence of the large ground finch; the smaller-beaked individuals of the medium ground finch may have been able to survive better due to a lack of competition over large seeds with the large ground finch.

So, the situation concerning natural selection is not as simple as usually presented in the textbooks or in the later deleted comment of the National Academy of Sciences, which I discussed in the first two posts in this series. Rather, there are obviously important open questions yet to be solved, including the possible (not even mentioned above) effects of hybridization between Geospiza magnirostris and G. fortis. 

Nevertheless, even if the competition hypothesis between these two species were true, it would change the “Sisyphean evolution of Darwin’s finches” only with regard to its length and the number of its steps to fulfill the Sisyphean cycle. 

The more extensive but misleading claim of Peter and Rosemary Grant is that “selection oscillates in a direction.” Even if this doubtful assertion were true, it would ultimately be irrelevant for the origin of primary species and higher systematic categories. A sieve (natural selection), after all, cannot create new grains.

Next, “Island Evolution of Species: Typogenesis, Typostasis or Typolysis?”

Editor’s note: This article was updated on November 30, 2020.

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beak sizebirdsDarwin's FinchesdroughtsGalápagos finchesGalápagos Finches seriesGalápagos IslandsGeospiza fortisGeospiza magnirostrisgrainsHMGA2Macroevolutionnatural selectionNature (journal)Peter and Rosemary Grantrainy seasonSangeet LamichhaneyseedssieveSisyphean evolutionspeciessubjunctive